In a head-up display (HUD) system deployed in a vehicle or other application, light from a display is projected onto the windshield and reflected to the eye of the user to create a virtual image located at a given distance outside the windshield. Light incident on the windshield from the HUD projector may have an arbitrary polarization of any combination of the S polarization and/or P polarization relative to the windshield. Fresnel equations describe the reflection and transmission of electromagnetic waves at the interface of the windshield and the surrounding medium to arrive at the reflection and transmission coefficients for waves parallel to the plane of incidence as P polarized waves, and perpendicular to the plane of incidence as S polarized waves.
When polarizing sunglasses are worn while driving a vehicle, the reflected S polarized light from the windshield, which is also the horizontal polarized light relative to the earth, is blocked or at least partially blocked. For typical HUD displays, for example laser based displays, light emitting diode (LED) displays, liquid-crystal displays (LCD), and so on, the reflection from the windshield predominantly reflects horizontally polarized light into the eye box, and this light is also blocked or at least partially blocked by the polarizing sunglasses. Thus, the same reason that polarizing sunglasses are beneficial to the user in blocking light reflected off of ground surfaces outside the vehicle also prevents the user from properly viewing an image projected by a head-up display. Furthermore, where the HUD display is a laser based display, the first and second reflections of the emitted laser beams off the first surface and the second surface of the windshield may interfere since the first and second reflections may have the same or nearly the same polarization and since the laser beams comprise coherent light, thereby further impeding the proper viewing of the virtual image projected by the display.
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